Much effort has recently been expended on producing glass by sol/gel processes. For a recent partial review of the field, see, for instance, D. R. Uhlmann et al, in Better Ceramics Through Chemistry, Materials Research Society Symposia Proceedings, Vol. 32, C. J. Brinker et al, editor, (1984), pp. 59-70.
Sol/gel methods for producing glass precursor material can be divided into techniques that form a gel using pre-existing colloidal particles (e.g., fumed silica), and into techniques that form a gel by hydrolysis and polymerization of appropriate chemical compounds. This application is primarily concerned with glass produced by techniques in the latter category, and by a combination of the two techniques. The techniques of primary concern herein will be collectively referred to as "polymerization" techniques.
Polumerization techniques may result in gel formation even though a classical sol may not have been formed in the process. It is customary, however, to refer to these processes as sol/gel processes. Typically, gels formed by polymerization techniques are among the monolithic gels. Such a gel may be pictured as a continuous three dimensional molecular network, or a network of bonded 2-5 nm diameter particles, having a sponge-like structure, with liquid occluded in the interstices of the "sponge".
Prominent among the known polymerization techniques is the alkoxide method which is reviewed, for instance, by S. Sakka, Treatise on Material Science and Technology, Vol. 22, Academic Press (1982), pp. 129-167. See also L. C. Klein et al., Soluble Silicates, ACS Symposium Series, Vol. 194, American Chemical Society (1982), pp. 293-304.
Recently a very convenient and effective sol/gel method of forming silica-based glass has been disclosed. See U.S. Pat. No. 4,767,429, issued Aug. 30, 1988 for J. W. Fleming and S. A. Pardenek, incorporated herein by reference. The method, termed the "Vapogel" technique, involves the introduction of a silicon halide-containing gas (e.g., SiCl.sub.4) into an aqueous medium such that gelation occurs. The resulting gel body typically is dried, and the thus produced porous silica-containing body typically either is sintered such that a glass body results, or is fragmented and the resulting particles used as feed stock in the manufacture of a glass body.
Various techniques are known for producing a glass body from material produced by the sol/gel method. Among these techniques are the double dispersion method of U.S. Pat. No. 4,419,115, and the particle fusion technique of U.S. Pat. No. 3,954,431, incorporated herein by reference, both co-assigned with this. See also S. Sudo et al, Technical Digest, Fourth International Conference on Integrated Optics and Optical Fiber Communication, Tokyo (1983), 27A3-4. The technique of the '431 patent comprises forming a glass body by fusing glass particles to a glass substrate by means of a plasma torch.
Among the advantages of sol/gel processes are their typically relatively low processing temperatures, and, frequently, their potential for economical production of high purity glass.
Among the articles that can potentially comprise a sol/gel-derived glass body is optical fiber drawn from a fiber preform. Such fiber typically is silica-based and comprises a core contactingly surrounded by a cladding, with the former having a higher refractive index than the latter to achieve guiding of electromagnetic radiation of an appropriate wavelength, e.g., in the range 0.7-1.6 .mu.m. The refractive index difference is produced, for instance, by incorporating an up-dopant (a dopant which increases the refractive index of SiO.sub.2, e.g. GeO.sub.2) into the core region and/or incorporating a down-dopant (a dopant which deceases the refractive index of SiO.sub.2 e.g., fluorine) into the cladding. Other examples of articles that can potentially comprise a sol/gel derived glass body are lenses and prisms, and high silicas glass tubes such as are used as substrate tubes in the MCVD process.
As discussed above, it is frequently necessary to form particles from the porous sol/gel-derived material. This tends to be a lengthy process that furthermore has the potential for contamination of the material. For instance, when producing relatively large (e.g. 25 liter) batches of Vapogel material, drying of the material, by heating it in a large container providing 2.5 Kw heating while drawing a vacuum on the container, may take as long as 24 hours, and frequently results in density variation in the dried material due to a difference in drying conditions experienced by the outer regions of the material as compared to the inner regions.
In U.S. patent application Ser. No. 940,392, filed Dec. 11, 1986, now U.S. Pat. No. 4,872,895, co-assigned with this and incorporated herein by reference, is disclosed a method of forming discrete, wet gel particles, essentially all of which have a predetermined, substantially uniform size. The particles are formed by mechanically subdividing, prior to or during gelation, a sol that is capable of undergoing gelation to yield a substantially cohesive gel body, or they are formed by mechanically subdividing a wet, cohesive, substantially elastic gel body (e.g., by pushing the gel body through a screen). In both of the above embodiments the particle size is at least in part determined by the size of a mechanical sizing element, e.g., the size of an orifice or of screen openings. In such processing care must be taken in handling to avoid contamination.
In view of the potential importance of sol/gel-derived glass bodies it would be highly desirable to have available a simple method that can quickly and efficiently produce particulate sol/gel-derived material without resulting in potential contamination thereof. In particular, it would be desirable to have available such a method that can produce particulate material having a relatively narrow size distribution, since particle fusion techniques are typically advantageously practiced with substantially uniformly sized particles. This application discloses such a method.